Electron microscopy (EM)

Electron microscopy by SIVASANGARI SHANMUGAM.
Electron microscopy is a technique for obtaining high-resolution images of biological and non-biological specimens.

1. SIVASANGARI SHANMUGAM ELECTRON MICROSCOPY

2. CONTENTS Introduction Types Principles Parts of Microscope How does it works? Applications Advantages Disadvantages

3. INTRODUCTION Electron microscopy is a technique for obtaining high resolution images of biological and non-biological specimens. It is used in biomedical research to investigate the detailed structure of tissues, cells, organelles and macromolecular complexes.  The high resolution of EM images results from the use of electrons (which have very short wavelengths) as the source of illuminating radiation.  Electron microscopy is used in conjunction with a variety of ancillary techniques (e.g. thin sectioning, immuno -labeling, negative staining) to answer specific questions. 

4. EM images provide key information on the structural basis of cell function and of cell disease.   Ernst Ruska (1906-1988), a German engineer and academic professor, built the first Electron Microscope in 1931, and the same principles behind his prototype still govern modern EMs. An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. It is a special type of microscope having a high resolution of images, able to magnify objects in nanometres , which are formed by controlled use of electrons in vacuum captured on a phosphorescent screen.

5. TYPES There are two main types of electron microscope. Transmission Electron Microscope (TEM) Scanning Electron Microscope (SEM) TEM - It is used to view thin specimens (tissue sections, molecules, etc) through which electrons can pass generating a projection image.  SEM - It depends on the emission of secondary electrons from the surface of a specimen. 

6. PRINCIPLES Electron microscope works on the principle of applying kinetic energy to produce signals on the interaction of the electrons. These electrons are secondary electrons, backscattered electrons and diffracted backscattered electrons which are used to view crystallized elements and photons. Secondary and backscattered electrons are used to produce an image. The secondary electrons are emitted from the specimen play the primary role of detecting the morphology and topography of the specimen while the backscattered electrons show contrast in the composition of the elements of the specimen.

7. Electron gun- The electron gun is a heated tungsten filament, which generates electrons. Condenser lens  - Focuses the electron beam on the specimen. A second condenser lens forms the electrons into a thin tight beam. Specimen Holder- The specimen holder is an extremely thin film of carbon held by a metal grid. Image viewing and Recording System- The final image is projected on a fluorescent screen. Below the fluorescent screen is a camera for recording the image.

8. PARTS OF ELECTRON MICROSCOPE Electron gun Electromagnetic lens Condenser lens Specimen holder Recording system

9. Objective lens - The electron beam coming out of the specimen passes down the second of magnetic coils called the objective lens, which has high power and forms the intermediate magnified image. Ocular lens- The third set of magnetic lenses called projector (ocular) lenses produce the final further magnified image. Each of these lenses acts as an image magnifier all the while maintaining an incredible level of detail and resolution.

10. HOW DOES IT WORKS? The electron gun generates electrons. Two sets of condenser lenses focus the electron beam on the specimen and then into a thin tight beam. To move electrons down the column, an accelerating voltage (mostly between 100 kV-1000 kV) is applied between tungsten filament and anode. The specimen to be examined is made extremely thin, at least 200 times thinner than those used in the optical microscope. Ultra-thin sections of 20-100 nm are cut which is already placed on the specimen holder. The electronic beam passes through the specimen and electrons are scattered depending upon the thickness or refractive index of different parts of the specimen.

11. HOW DOES IT WORKS? The denser regions in the specimen scatter more electrons and therefore appear darker in the image since fewer electrons strike that area of the screen. In contrast, transparent regions are brighter. The electron beam coming out of the specimen passes to the objective lens, which has high power and forms the intermediate magnified image. The ocular lenses then produce the final further magnified image.

12. APPLICATIONS Electron microscopes are used to investigate the ultra structure of a wide range of biological and inorganic specimens including microorganisms, cells, large molecules, biopsy samples, metals, and crystals. Industrially, electron microscopes are often used for quality control and failure analysis. Modern electron microscopes produce electron micrographs using specialized digital cameras and frame grabbers to capture the images. Science of microbiology use EM to study of microorganisms like bacteria, virus and other pathogens have made the treatment of diseases very effective.

13. ADVANTAGES Very high magnification Incredibly high resolution Material rarely distorted by preparation It is possible to investigate a greater depth of field Diverse applications

14. DISADVANTAGES The live specimen cannot be observed. As the penetration power of the electron beam is very low, the object should be ultra-thin. For this, the specimen is dried and cut into ultra-thin sections before observation. As the EM works in a vacuum, the specimen should be completely dry. Expensive to build and maintain Requiring researcher training Image artifacts resulting from specimen preparation. This type of microscope is a large, cumbersome extremely sensitive to vibration and external magnetic fields.

15. REFERENCES https://microbenotes.com/electron-microscope-principle-types-components-applications-advantages-limitations/#:~:text=An%20electron%20microscope%20is%20a,captured%20on%20a%20phosphorescent%20screen . https://bitesizebio.com/29197/introduction-electron-microscopy-biologists/ https://www.umassmed.edu/cemf/whatisem/

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